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.Dd November 9, 2011
.Dt FLOCK 2
.Os
.Sh NAME
.Nm flock
.Nd "apply or remove an advisory lock on an open file"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/file.h
.Fd "#define   LOCK_SH        0x01      /* shared file lock */"
.Fd "#define   LOCK_EX        0x02      /* exclusive file lock */"
.Fd "#define   LOCK_NB        0x04      /* do not block when locking */"
.Fd "#define   LOCK_UN        0x08      /* unlock file */"
.Ft int
.Fn flock "int fd" "int operation"
.Sh DESCRIPTION
The
.Fn flock
system call applies or removes an
.Em advisory
lock on the file associated with the file descriptor
.Fa fd .
A lock is applied by specifying an
.Fa operation
argument that is one of
.Dv LOCK_SH
or
.Dv LOCK_EX
with the optional addition of
.Dv LOCK_NB .
To unlock
an existing lock
.Dv operation
should be
.Dv LOCK_UN .
.Pp
Advisory locks allow cooperating processes to perform
consistent operations on files, but do not guarantee
consistency (i.e., processes may still access files
without using advisory locks possibly resulting in
inconsistencies).
.Pp
The locking mechanism allows two types of locks:
.Em shared
locks and
.Em exclusive
locks.
At any time multiple shared locks may be applied to a file,
but at no time are multiple exclusive, or both shared and exclusive,
locks allowed simultaneously on a file.
.Pp
A shared lock may be
.Em upgraded
to an exclusive lock, and vice versa, simply by specifying
the appropriate lock type; this results in the previous
lock being released and the new lock applied (possibly
after other processes have gained and released the lock).
.Pp
Requesting a lock on an object that is already locked
normally causes the caller to be blocked until the lock may be
acquired.
If
.Dv LOCK_NB
is included in
.Fa operation ,
then this will not happen; instead the call will fail and
the error
.Er EWOULDBLOCK
will be returned.
.Sh NOTES
Locks are on files, not file descriptors.
That is, file descriptors
duplicated through
.Xr dup 2
or
.Xr fork 2
do not result in multiple instances of a lock, but rather multiple
references to a single lock.
If a process holding a lock on a file
forks and the child explicitly unlocks the file, the parent will
lose its lock.
.Pp
The
.Fn flock ,
.Xr fcntl 2 ,
and
.Xr lockf 3
locks are compatible.
Processes using different locking interfaces can cooperate
over the same file safely.
However, only one of such interfaces should be used within
the same process.
If a file is locked by a process through
.Fn flock ,
any record within the file will be seen as locked
from the viewpoint of another process using
.Xr fcntl 2
or
.Xr lockf 3 ,
and vice versa.
.Pp
Processes blocked awaiting a lock may be awakened by signals.
.Sh RETURN VALUES
.Rv -std flock
.Sh ERRORS
The
.Fn flock
system call fails if:
.Bl -tag -width Er
.It Bq Er EWOULDBLOCK
The file is locked and the
.Dv LOCK_NB
option was specified.
.It Bq Er EBADF
The argument
.Fa fd
is an invalid descriptor.
.It Bq Er EINVAL
The argument
.Fa fd
refers to an object other than a file.
.It Bq Er EOPNOTSUPP
The argument
.Fa fd
refers to an object that does not support file locking.
.It Bq Er ENOLCK
A lock was requested, but no locks are available.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr dup 2 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr fork 2 ,
.Xr open 2 ,
.Xr flopen 3 ,
.Xr lockf 3
.Sh HISTORY
The
.Fn flock
system call appeared in
.Bx 4.2 .
